FIGS. 8(a)-8(c) show a method of producing an aperture for a gate of an HEMT (High Electron Mobility Transistor) described in Electronics Letters, volume 24, 1988, page 1327, by Hanyu et al.
As shown in FIG. 8(a), an SiO.sub.2 film 2a is deposited on a compound semiconductor substrate 1 by plasma CVD (Chemical Vapor Deposition) or sputtering. Next, a photoresist 4 is deposited and formed into a photoresist pattern. An optical exposure or an EB (electron beam) exposure is used for patterning of the photoresist 4 and an appropriate photoresist is used depending on the exposure method used. EB exposure is used by Hanyu. Thereafter, as shown in FIG. 8(b), the SiO.sub.2 film 2a is etched by RIE (Reactive Ion Etching) using the photoresist pattern as a mask. Afterwards, as shown in FIG. 8(c), the photoresist 4 is removed so that the SiO.sub.2 film 2a has an aperture. In Hanyu, metal is deposited on the entire surface of the substrate by sputtering. Thus, a gate having a Schottky barrier comprising the metal and the semiconductor is produced at the aperture for the HEMT.
FIGS. 9(a)-9(c) show a triple-layer photoresist production method used for producing a dummy pattern for a gate in a SAINT (Self-Aligned Implantation for N.sup.+ -layer Technology) MESFET (Metal Semiconductor Field Effect Transistor) described in IEEE Transactions, volume ED-29, 1982, page 1772, by Yamasaki et al.
As shown in FIG. 9(a), a lower layer photoresist 7 is deposited on a compound semiconductor substrate 1 and an SiO.sub.2 film 9 is deposited thereon by sputtering. An upper layer photoresist 8 having a higher resolution than that of the lower layer photoresist 7 is then deposited. Next, as shown in FIG. 9(b), the sputtered SiO.sub.2 film 9 is etched by RIE using, for example, a mixture of CF.sub.4 and O.sub.2, using the upper layer photoresist 8 as a mask. Finally, as shown in FIG. 9(c), RIE using, for example, O.sub.2 and the sputtered SiO.sub.2 film 9 as a mask, removes the lower layer photoresist 8.
In this technique, the pattern of the upper layer photoresist 8, which has a low anti-RIE property but high resolution, is transferred to the lower layer photoresist 7 which has high thermal stability, a strong anti-RIE property, and a large thickness. The thermal stability and anti-RIE property of the lower layer photoresist 7 are utilized in a later process step. Furthermore, processing requiring a large film thickness and high resolution can be carried out.
The prior art production methods described above have the following problems.
In the method of producing an aperture shown in FIG. 8(c), when the SiO.sub.2 film 2a is etched by RIE, the photoresist 4 is also etched, thereby increasing the size of the aperture. When the aperture is used to form the gate of an FET, the gate length is increased by etching of the photoresist 4. Even when the photoresist 4 has an aperture size of 0.2 micron produced by EB exposure, the aperture size of the SiO.sub.2 film 2a after RIE increases to about 0.3 micron. This reduces the transconductance, gm, of the transistor.
In the method shown in FIGS. 8(a)-8(c), the exposed part of the surface of the semiconductor substrate 1 is damaged when the SiO.sub.2 film 2 is etched by RIE. The damage below the gate of the transistor reduces the controllability of transconductance and the threshold voltage, V.sub.T.
In the triple-layer photoresist production method shown in FIGS. 9(a)-9(c), when the sputtered SiO.sub.2 film 9 is etched by RIE using the upper layer photoresist 8 as a mask, the photoresist 8 is also etched and the pattern of the sputtered SiO.sub.2 film 9 becomes smaller than that of the upper layer photoresist 8. When the lower layer photoresist 7 is etched by RIE with O.sub.2 gas and using the sputtered SiO.sub.2 film 9 as a mask, the pattern of the photoresist 7 becomes still smaller than that of the sputtered SiO.sub.2 film 9. Therefore, this method for producing a narrow gate requires RIE conditions having quite high selectivity, resulting in processing difficulties. The gate length of a SAINT MESFET is typically about 0.3 micron and it is difficult to further reduce the size at the present time.